Forests and CO2 — It’s complicated!

One of the few lodgepole seedlings to survive the industrial clearcutting on the north shore of the Frisco Peninsula.
Climate models may be overestimating the carbon-capturing capacity of forests. @bberwyn photo.

Loss of nitrogen a key factor in forest equation

Staff Report

Forests may grow faster as atmospheric CO2 increases, but that doesn’t mean they’ll absorb more of the heat-trapping gas. Instead, a shortage of nitrogen means plants won’t be able to fix as much carbon as projected by some climate models.

“Forests take up carbon from the atmosphere, but in order for the plants to fix the carbon, it requires a certain amount of nitrogen,” said researchers Prasanth  who took a close look at the chemistry of secondary forests that are regrowing after deforestation, wood harvest and fires.

“If that ratio of carbon to nitrogen isn’t right, even if you add many times more carbon than it gets currently, the forests cannot absorb the extra carbon,” Meiyappan said.The new study was conducted by scientists with the University of Illinois at Urbana-Champaign and the University of Bristol in the UK. The results suggest that current climate models probably overestimate the amount of carbon capture by new forests.

“Most forested land on Earth is a secondary forest,” said atmospheric science expert professor Atul Jain. “When the original forested land is disturbed, much of the nitrogen that is in the soil is released into the atmosphere, making growth in these areas slow-going.”

“The carbon that is lost from a forest during harvest or fires can be replaced over time if the forest regrows, so net carbon loss is minimal. If forest regrowth is limited due to a lack of nitrogen, then net carbon emissions are higher,” said University of Bristol geography lecturer Joanna House.

Another issue compounding nitrogen limitation is the removal of plant matter for burning. The Intergovernmental Panel on Climate Change has projected that bioenergy will be needed to meet an anticipated dramatic increase in the world’s energy consumption while still limiting carbon dioxide emissions. Forest products and residues – the leaves and sticks below the canopy – and an increase in the range of timber-harvested forests will be a large factor in meeting those energy demands.

“Residue decomposes over time and releases nitrogen for the plants,” Jain said. “By taking it out, it further tilts the nitrogen-to-carbon ratio in the future.”

The team found that because most climate models included in the IPCC projections don’t take into account the effect of nitrogen, they underestimate 21st century net carbon emissions in the land sector by at least 90 percent, and by as much as 150 percent.

“If net land-based emissions are underestimated, it means stronger mitigation action would need to be taken in other sectors such as energy to meet the same mitigation targets,” House said

Jain said the panel’s latest report discussed the importance of how the nitrogen and carbon cycles interact, but this is the first study to quantify how that interaction affects emissions as land cover and land use change.

“We hope that this study will trigger a discussion on the importance for climate models to consider nitrogen limitation when evaluating the strength of future land carbon uptake, for the next IPCC report,” Meiyappan said. He added that when there are more models, the panel can take an average across them and come up with the most likely and relevant emissions projections.

The paper was published in the journal Global Biogeochemical Cycles.

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